A high-performance air filter which has a folded filter medium made from glass fiber through a wet process provides clean air, and thus has contributed to the semiconductor industry. Also, high integration of LSI has been developed, and minimum pattern dimensions of a device have been reduced. Thus, the size of fine particles which should be removed has been decreasing. Accordingly, ULPA (Ultra Low Penetration Air) filters having greater performance than HEPA (High Efficiency Particulate Air) filters now are often used in clean rooms for semiconductor manufacture.
However, borosilicate glass fiber used in a filtration medium of a conventional ULPA filter is attacked by hydrofluoric acid, which is used in manufacturing semiconductor, and BF.sub.3 gas is generated. The boron accumulates on a silicon wafer in the manufacturing process of semiconductor, thus causing abnormal characteristics. Thus, it has become an urgent matter to deal with this problem.
Recently, particular attention has been paid to a high-performance air filter using a filter medium made from polytetrafluoroethylene (hereinafter referred to as "PTFE") which does not generate boron (e.g. Japanese Publication of Unexamined Patent Application (Tokkai) No. HEI 5-202217, or WO94/16802).
Such an air filter made from PTFE is free of the problem of off-gassing of boron etc. because the PTFE used in the filter medium is very clean and has excellent chemical resistance. Furthermore, the air filter made from PTFE also can achieve the same or higher collection efficiency of floating fine particles as the above-mentioned ULPA filter made from glass fiber. Moreover, because the PTFE air filter displays a considerably lower pressure loss than the glass fiber filter when they have the same collection efficiency (for example, the pressure loss of a ULPA filter made from PTFE is two third that of a ULPA filter made from glass fiber), the energy cost for the operation of the filter can be reduced. Because of these characteristics, the use of an air filter made from PTFE is spreading rapidly into various industries such as the semiconductor industry.
However, the requirement for cost reduction is severe in many industries. Therefore, it is desired to further develop a PTFE filter medium that satisfies the conditions "further improved collection efficiency and a lower pressure loss to reduce the cost for the filter operation" at the same time and an air filter unit using such a filter medium (not limited to ULPA type, but also including HEPA type and a medium-performance type).
However, an increase in collection efficiency and a reduction in pressure loss contradict each other (see, for example, Nitto Giho, Vol. 34, No. 1 (May, 1996)). Thus, it is not easy to solve the above-mentioned problems, and a satisfactory product has not yet been obtained.
Also, as is described in the above-mentioned Japanese Publication of Unexamined Patent Application (Tokkai) No. HEI 5-202217 or WO94/16802, for example, it is considered that when a conventional PTFE filter medium is used as a high-performance air filter, the aim of the filter cannot be attained unless its average pore diameter is in the range of 0.2 to 0.5 .mu.m. However, it is not easy to control the average pore diameter within this very small and narrow range at all times.